This invention relates to a rotary heat exchanger and more particularly to a rotary dryer having a hollow shaft and a number of hollow vanes or paddles spaced along the shaft for receiving and circulating a heat exchange fluid.
Rotary heat exchangers are well known in the art. For example, the U.S. Pat. No. 3,020,025 of Richard F. O'Mara discloses a longitudinally extending trough or casing and a double hollow shaft assembly extending the length of the casing. Typically, the casing includes an inlet opening at one end thereof for receiving material which is to be heated and/or dried. And then, a discharge port is provided at the other end.
The shaft assembly includes a central portion for the axial passage of a heat transfer fluid such as steam. And spaced longitudinally along the shaft are a number or radially projecting vanes. The vanes are adapted to receive steam or other condensing vapor from the hollow shaft and upon rotation, to heat and move material from the inlet area of the casing towards the discharge port.
There have been a number of approaches for improving the heat transfer or energy efficiency in rotary heat exchangers. For example, the U.S. Pat. No. 3,613,777 of Jiyuichi Nara discloses one such approach. Nara discloses a hollow shaft and a longitudinal separator in the center of the shaft. The heating medium passes through a first chamber formed by the separator and into a vane by means of a tubular conduit. The steam in the vane condenses and water is drained into a second chamber in the shaft.
Nara also discloses a shaft with no separator and the use of steam when water of condensation is small in quantity. In that case communication tubes are formed inside the shaft in a somewhat protruding manner. By so doing, when the revolving body (vane) is rotated and takes its lower position, the steam enters the revolving body through the communication tubes and when the body takes its upper position, water of condensation enters the shaft through communication holes and is discharged from the bottom of the shaft.
It has now been found that rotary dryers made in accordance with the present invention have improved thermal efficiency. In essence, they have improved thermal efficiency because of the effective removal of liquid condensate from the vanes and because any liquid condensate in the hollow shaft is prevented from entering the vanes when the vanes are in their lower position, i.e. disposed below the level of the shaft.
In addition, the rotary dryers disclosed herein have means for directing a condensing heat transfer fluid such as steam into the vanes. They also have means for maintaining the level of liquid condensate in the hollow shaft at less than a predetermined level. And, the dryers can be constructed at an economical cost and in a manner that does not weaken their structural integrity.
Such dryers are thought to be particularly applicable for drying sewage sludge. For example, sewage sludge can be dewatered by centrifuge, vacuum filter or in specially constructed hanging bags to about 20% solid content. This dewatered sludge or sewage is then dried in a rotating dryer according to the present invention. And the dried sludge can then be burned in a multiple hearth furnace with a waste-heat boiler or the like. Steam generated from the incineration can then be used as the heat transfer fluid for the rotary dryer.